1. Field of the,Invention
The present invention relates to a degradable foam. More particularly, the invention relates to a foam which is composed of a thermoplastic polymer Composition substantially comprising polylactic acid or a copolymer of lactic acid and hydroxycarboxylic acid and has degradable characteristics in the natural environment.
The foam of these thermoplastic and degradable polymers can be used for food containers and throwaway cups after processing into sheets or used for heat insulating materials or cushioning materials in the form of bulk.
2. Related Art of the Invention
Foams of polyolefin, polyurethane and polyamide based resin have been conventionally used in a broad field of containers, cushioning materials and heat insulating materials. These foams, in particular, can be processed into formed articles with a small amount of resin and hence are used for food containers, disposable cups and lunch boxes which are cheap and light and also for interior dividers and wrapping materials of readily damageable matters such as vegetables, fruits and perishable foods.
However, the foams prepared from these resins or articles such as containers, cushioning materials and heat insulating materials which are prepared from these foams are of great bulk on discarding and form an increasing amount of garbage. Additionally, conventional products have very slow degradation speed in the natural environment and remain semipermanently when disposed under the ground. Further, throw away plastics have caused problems such as damage of a view and destruction of the living environment of marine organisms.
However, the foams capable of readily degrading in the natural environment and articles such as containers, cushioning materials and heat insulating materials obtained by using these foams have not yet been known.
On the other hand, a lactic acid-based polymer, that is, a polymer substantially consisting of lactic acid or a copolymer of lactic acid has been known as a thermoplastic and degradable polymer.
The lactic acid-based polymer has degradable characteristics in the natural environment. When the polymer is allowed to stand in soil or sea water, the polymer starts degradation within several weeks and disappears in about a year. Final degradation products are lactic acid, carbon dioxide and water. All of these products are nonhazardous.
The raw material lactic acid can be obtained by fermentation of inexpensive materials such as corn starch and corn syrup and can also be prepared from petrochemicals such as ethylene.
U.S. Pat. No. 1,995,970 discloses a preparation process by polymerization of lactic acid, lactide or a mixture of these compounds. Polymers of lactic acid are usually prepared from a cyclic dimers of lactic acid, called lactide. The polymers can also be directly prepared from lactic acid through dehydrating condensation. The straight chain polyester prepared from lactide by ring-opening polymerization has a high molecular weight. On the other hand, the polymer directly obtained from lactic acid by dehydrating condensation is liable to difficultly increase molecular weight even through reaction time is extended.
The lactic acid-based polymer has been used for surgical sutures and medical slow release matrixes because of good biological compatibility and degrading ability. However, foams of the polymer which are used for containers, heat insulating materials and cushioning materials have not yet been known.